Zeolite, known as a compound with a crystalline microporous structure, that is, aluminosilicates represented by the following general formula include cations with a large ion exchange capacity and a three dimensional net-like structure and have a shape and a size of the cavity and channel unique to crystals thereof.(M1,M21/2)m[AlmSinO2(m+n)]·xH2)
(In the formula, M1 indicates a monovalent cation such as Na+, K+, M2 indicates a divalent cation such as Ca++, Sr++, and m is equal or smaller than n, and x is indefinite.)
Microporous crystals such as the zeolite, have specific functions such as adsorption action, ion exchange action, as characteristics based on the cavity structure and chemical composition, and are used in application for molecular sieves, cluster encapsulation or catalyst carriers, etc., and in addition to these applications, engineering applications of microporous crystals are being attempted in various fields such as electronic devices and sensors. Porous silica crystals of the present invention are one kind of zeolites and one comprising only silicon and oxygen atoms, in which a typical example is silicalite.
The zeolites have nanosized orderly pores and formation of a semiconductor, electroconductive polymer, etc., within the pores allows to demonstrate quantum specific physical properties as electronic devices and optical devices. When used as a sensor, they can be expected to show high selectivity and high response.
However, zeolites, etc., are generally a few micrometers or less in crystal size, and the crystals thereof are difficult to be arranged in order. When used in semiconductor elements, etc., orderly arrangement of the crystals is important and each of the crystals also has to be similarly sized. In this regard, with zeolites having a crystal size of at least 0.5 mm, preferably a few millimeters, an element having any size and shape with one side of 0.5 mm or more can be easily fabricated.
In a sensor element, a dramatic increase in an absorbtion selectivity can be expected if large crystals having size of 0.5 mm or higher and therefore, having a substantially lower proportion of the outer surface are used, since the smaller the outer surface area showing no absorption selectivity is, the higher the selectivity becomes.
A synthetic method of a large-size single crystal of the porous silica crystals includes one using a bulk material (for example, refer to Patent Document 1). This describes that fused quartz or ceramics, etc., can be used as a bulk material to synthesize a huge crystal with several hundred micrometers or larger through a hydrothermal reaction.
In Patent Document 1, cutouts of fused quartz are used as the bulk material to fill in a pressure resistant vessel to perform the hydrothermal reaction.
Patent Document 1: Japanese Patent Laid-Open No. 2000-34188